Conventionally, there has been known an image reading apparatus which is mounted on a copying device, a scanner device or a multi function device having a copy function and a scanner function and comprises an automatic document delivering device referred to as an ADF (Auto Document Feeder) for delivering a document from a sheet feed tray to a sheet discharge tray through a delivery path. Moreover, there has been known an automatic document delivering apparatus For switching back a document having both sides, that is, first and second faces, which are printed, in the middle of a delivery to invert a leading and a trailing end in the document, thereby carrying out a delivery for reading both sides for the document in order to read the document (for example, see Japanese Patent No. 3546822).
FIG. 19 shows a delivery path for a conventional image reading apparatus comprising an ADF capable of reading both sides. As shown in FIG. 19, a document P having a first face (a first page) mounted upward in a sheet feed tray 100 is fed to a delivery path 102 by means of a sheet feed roller 101. In the delivery path 102, the document P is delivered to a delivery roller 103 provided properly and the first face of the document P is read by image reading means such as a CCD or a CIS when passing through a reading position X. When a sensor detects a trailing end of the document P having the first face read, a sheet discharge roller 104 is stopped in such a state as to nip the vicinity of the trailing end of the document.
As shown in FIG. 20, when the sheet discharge roller 104 is rotated reversely, the document P is delivered to a switchback path 105. The document P advances to an upstream side of the reading position X of the delivery path 102 from the switchback path 105 again. Consequently, the leading and the trailing end of the document Pare inverted. When the document P is delivered by means of the delivery roller 103 and passes through the reading position X, a second face of the document P is read by the image reading means. When the sensor detects the trailing end of the document P having the second face read, the sheet discharge roller 104 is stopped in such a state as to nip the vicinity of the trailing end of the document P again and the document P is then fed reversely through the switchback path 105. The document P advancing to the delivery path 102 from the switchback path 105 again is brought into a state in which the leading and the trailing end are inverted again, that is, a state in which the first face is opposed to the reading position X. Thereafter, the document P is delivered through the delivery path 102 and the first face is turned downward so that the document P is discharged to a sheet discharge tray 106. Consequently, both the first and second faces of the document P are read, and furthermore, the document P is discharged to the sheet discharge tray 106 in loading order onto the sheet feed tray 100.
In such an image reading apparatus comprising the ADF, it has been known that there is generated a so-called skew in which the document is tilted with respect to a delivery direction when the document is to be delivered. There are various causes for the skew, that is, a tilt is generated when the document is to be mounted on the sheet feed tray 100, an individual variation in the sheet feed roller 101 and the delivery roller 103 is generated, and a difference in a degree of smoothness of the surface and back face of the document is made. In the double side reading operation, the delivery distance of the document is increased so that the skew is generated easily. In some cases in which the degrees of smoothness of the surface and back face of the document are different from each other, moreover, the skew is easily generated on one of the surface and the back face.
For example, in the case in which the skew is not generated when the image of the first face of the document is to be read but is generated when the image of the second face is to be read as shown in FIG. 21, image data having different degrees of the tilt of the image are created on the surface and back face of the document. If both sides are copied by using such image data, the degrees of the tilt of the image are different from each other between the surface and the back face for each recording sheet. Consequently, there is a problem in that an image for each page has no unification and a poor impression is given.